In conventional telephone switching systems the subscribers are interconnected by the metal contacts of the switching grid. However, the replacement of these metal contacts by electronic switches, present new problems. In particular, these switches have a higher resultant resistance than the corresponding metal contacts, which they are replacing.
In a conventional telephone set the energy imparted to the microphone by the voice of the user is applied to a hybrid transformer which transfers part thereof to the telephone line itself and the remaining part to a balancing circuit. When the impedance of this balancing circuit exactly matches that of the line, part of the energy will be transferred to the telephone receiver; this is referred to as the side tone effect. Although this transfer is necessary, it should not exceed a suitable level because, as is well known, the higher the intensity of the acoustic energy being transferred, to the speakers ear the more softly the user will tend to speak. In order to keep this effect within reasonable limits, the characteristics of the balancing circuit are selected as a function of the impedance of the line to which the telephone set is connected. Since conventional telephone sets are generally designed to be connected to common-carrier lines and central offices or exchanges, the impedance of the balancing circuit incorporated therein is made to match that of said lines and central offices; this circuit usually consists of a resistance-capacitance network.
When a telephone set incorporating such a circuit is used in a private automatic branch exchange (PABX), the switching network of the PABX must be perfectly "transparent" if the circuit is to correctly perform its intended function. In providing satisfactory transmission characteristics to signal transmission through two wire transmission lines, it is often necessary to introduce means for compensating the available signal levels.
A circuit generally referred to as a 22-type phone line repeater is used to provide transmission level compensation. Due to application differences, the circuit structures are different. A line repeater requires complex adjustable networks to achieve proper balance whereas this circuit uses a bridge with fixed component values. The bidirectional amplifier mirrors impedance changes from one side to the other side and therefore is transparent to the phone and the phone line. A 22-type line repeater would have adjustable networks to compensate for line impedance changes.